Edit rust project 4 code

[brson]

Subtask of #92.

After you have written your own version of the project, then read the example project, including the test cases.

The example project needs to represent the very best of Rust coding practices. The test cases especially are the real "specification" for the project, and students will be reading them to figure out what exactly their own code is supposed to be doing. Test code is often written with less care than production code. That will not be the case in this project.

Considerations:

• Code should be idiomatic, but also straightforward. It shouldn't be unnecessarily clever.
• Public items should be documented, without simply repeating the item name. The crate should contain the attribute to deny undocumented items.
• Code should be rustfmted.

As with the text, each project's code needs to be consistent from project to project. Functions with identical specs should be implemented and formatted the same. They should be ordered in the same sequence.

For test cases, each project should contain the previous project's tests, in a consistent and sensible order.